The formation of a mature neuromuscular junction (NMJ) requires changes in expression of several muscle genes that are regulated by activation of the nicotinic acetylcholine receptor (ChR). During development, after its insertion into the NMJ the AChR undergoes a series of physiological and structural changes that have been ascribed to the replacement of the gamma (gamma) subunit by the epsilon (epsilon) subunit. These changes may have a role in protecting muscle from potentially excitotoxic effects of excessive stimulation by acetylcholine (ACh) since they lead to a diminution of the ACh-sensitivity of muscle fibers just as the nerve has arrived to establish cholinergic transmission. Failure to make this subunit switch might be the molecular basis for the slow channel syndrome, a hereditary muscle disease. The goal of this study is threefold: To investigate 1) the the mode of regulation of the gamma to epsilon AChR subunit switch and its role in the developmental changes of the AChR of the NMJ; 2) the influence these changes may have on regulation of cholinergic genes and other proteins involved in synaptic transmission; 3) the role of these changes in maintaining the integrity and long-term viability of neuromuscular transmission. This latter goal will test the hypothesis that the phenomenon of excitotoxicity can arise as a result of an abnormal response on the part of an excitatory neurotransmitter receptor. For this study several lines of transgenic mice have been generated that constitutively express the mRNA for the embryonic (gamma) subunit of AChR in muscle. By studying the transcription and stability of the mRNA for gamma, the synthesis of the gamma subunit protein, and its assembly into mature AChRs, the control system for regulation of the embryonic phenotype will be clarified. By studying the properties of the gamma-containing AChRs incorporated into adult motor endplates, the role of the subunit switch in the developmental changes of AChR will be determined. The effect of this persistence of fetal properties on both muscle viability and expression of other genes involved in cholinergic transmission will be evaluated. Further understanding of the molecular basis for the developmentally related phenotypic changes in AChRs and the NMJ will broaden insights into the basis for such changes in other neurotransmitter systems and in neural control in general. Pathologic changes demonstrated in the muscle of animals with such abnormal AChRs would implicate genes for excitatory neurotransmitter receptors in the pathogenesis of hereditary neurodegenerative diseases. It may also provide a model of excitotoxicity for investigation of the second messenger systems participating in its pathogenesis and for development of possible drug therapies.